With unstable power supply, the capacitor
can be used as a backup power supply to support the MCU to operate for a certain period of
time until the power supply is restored, or to provide a certain amount of time to save the
MCU's data.
In the printer cartridge, the three-wire
system cannot meet the MCU communication needs, as UART need two wires, VCC one and Ground
one. Here we can multiplex the VCC end with the TXD of the UART. While TXD is high, it can
charge capacitor as well as supply power for MCU. And as TXD goes to LOW, a diode can
separate the VCC terminal of TXD from the MCU, and rely on the capacitor to power the
MCU.